PROJECT SUMMARY/ABSTRACT Congenital heart disease (CHD), the most common human birth defect, is the result of abnormal development of discrete sub-types of cardiogenic cells during early embryogenesis. The recent ability to analyze the transcriptomes of tens of thousands of individual cells has made it tractable to discern the consequences of genetic mutation on small subsets of cells that could lead to CHD. Nearly 30% of all CHD involve the developing valvuloseptal region, which arise from some of the more rare cell types in the heart. Secreted myocardial signals in valve-forming regions are received by the endocardium and stimulate endocardial to mesenchymal transformation (EMT) resulting in cells that invade the extracellular matrix and form the primordia of future valves and contribute to cardiac septation. We performed single cell RNA sequencing (scRNAseq) of over 100,000 cells from mouse hearts between E7.75 ? E13.5 before and after EMT is occurring. We have, for the first time, defined the transcriptomes of specific individual cell types that are involved in valvuloseptal formation, revealing novel markers and pathways involved in this process. We have begun to define the pathways disrupted in the relevant cell types in a mouse model of atrioventricular septal defects (AVSD) caused by heterozygosity for Gata4 and Tbx5, and human iPSCs heterozygous for a GATA4 missense mutation from a patient with AVSD. AVSDs are most commonly seen in the setting of Trisomy 21 (Down Syndrome), yet the cause remains unknown. We propose to use the ?atlas? of single cell gene expression during valvuloseptal development and human iPSCs to test the hypothesis that the genetic pathways disrupted in specific cardiac cells in Down Syndrome overlap with those involving known regulators of atrioventricular septation and EMT, and that this knowledge will reveal the gene(s) involved in cardiac defects associated with Down Syndrome. We approach this through the following specific aims: 1) To determine cell types and gene networks involved in AVSD by comparing and contrasting temporal single cell gene expression and epigenetic networks disrupted in atrioventricular canal myocardium, endocardial cells, and valve mesenchymal cells in mouse models of AVSD; 2) To determine cell types and gene networks dysregulated in human AVSD disease models by comparing and contrasting single cell gene expression and epigenetic alterations in human iPSC-derived myocardial and endothelial cells from Trisomy 21 patients and from GATA4/TBX5 mutant cells; and 3) To determine which gene(s) on human chromosome 21 are sufficient to cause gene dysregulation observed in valvuloseptal defects and phenotypic changes in mice. The combination of mouse models, human iPSC models, and single cell genomics will reveal the cell types and mechanisms associated with valvuloseptal defects common to Down Syndrome.